A few weeks ago I was looking around the internets for information specifically about Mullard rectifiers, and came across this:
https://www.ebay.com/itm/252456585587It is more or less the motherload of tube data and info. It has tube manuals from pretty much all the players in the US, UK and Europe from the 30's (or maybe earlier) through the 70's, and tube amp and circuit books as well. 300 separate .PDF documents on a DVD for about 15 US$. I have only started looking at it, but so far the scans seem high quality, and for a nickel a book how can you go wrong?
(answer: you can't)
No relationship with the seller, just a happy tube geek and satisfied customer.